A comprehensive review of the pyrolysis process: from carbon nanomaterial synthesis to waste treatment

Abstract

Thermally induced chemical decomposition of organic materials in the absence of oxygen is defined as pyrolysis. This process has four major application areas: (i) production of carbon materials, (ii) fabrication of pre-patterned micro and nano carbon-based structures, (iii) fragmentation of complex organic molecules for analytical purposes and (iv) waste treatment. While the underlying process principles remain the same in all cases, the target products differ owing to the phase and composition of the organic precursor, heat-treatment temperature, influence of catalysts and the presence of post-pyrolysis steps during heat-treatment. Due to its fundamental nature, pyrolysis is often studied in the context of one particular application rather than as an independent operation. In this review article, an effort is made to understand each aspect of pyrolysis in a comprehensive fashion, ensuring that all state-of-the-art applications are approached from the core process parameters that influence the ensuing product. Representative publications from recent years for each application are reviewed and analyzed. Some classical scientific findings that laid the foundation of the modern-day carbon material production methods are also revisited. In addition, classification of pyrolysis, its history and nomenclature and the plausible integration of different application areas are discussed.

pyrolysis, carbon nanomaterial, pyrolytic graphite, glass-like carbon, micro/nano devices, waste treatment

INTRODUCTION

Pyrolysis is the key process in carbon nanomaterial synthesis [1–4], bulk carbon production [5, 6], fabrication of carbon-based devices [7–10], fuel generation from organic waste [11–13] and molecule fragmentation for their analysis via gas chromatography–mass spectroscopy (GC–MS) [14–16]. Primary examples of the technologically significant carbon materials prepared via pyrolysis include graphene [1, 17, 18], carbon nanotubes (CNTs) [1, 19, 20], carbon fibers (CFs) [1, 21–23], diamond-like carbon (DLC) coatings [2, 24] and other industrial carbons such as glass-like carbon (GC) [7] and graphite [5].

Unlike other manufacturing materials such as metals, the production of carbon relies heavily on synthetic routes. In fact, certain carbon allotropes (e.g. GC) are exclusively synthetic. Majority of carbon manufacturing pathways are based on pyrolysis, where a suitable organic precursor is heated to elevated temperatures in an inert environment and in some cases, a catalyst. This leads to a thermal decomposition of the precursor and the release of non-carbon atoms in various forms. Owing to carbon’s high thermal stability (in the absence of oxidants), some fraction of solid carbon is always obtained as a residue or in the form of smoke. This reactive solid carbon can potentially adopt numerous microstructural configurations, depending upon the type of precursor, its decomposition pattern, applied pressure (if any), the formation pathways of the larger carbon moieties and the thermodynamic stability of pyrolysis products [12, 25]. This principle has been used for manufacturing various crystalline as well as disordered carbons for several decades [26, 27]. With the advent of nanotechnology, the pyrolysis process and precursors have been optimized to yield the nano-scale one- and two-dimensional carbon structures. Modern-day chemical vapor deposition (CVD) technique designed for carbon nanomaterial synthesis is indeed based on the principle of pyrolysis [1, 28]. Interestingly, the primary reason behind the popularity of carbon in nanotechnology is the ease with which different carbon nanoforms can be deposited onto a range of substrates [6]. Needless to say, optimization of pyrolysis process is of utmost importance to anyone working in the field of carbon materials and associated technology.

Some large-scale industrial carbon materials such as graphite can be procured via mining. However, their synthetic production yields a high purity material and is therefore preferred. Pyrolysis additionally offers the possibility of tuning the microstructure of the ensuing carbon (e.g., enhancement of graphitic content [29, 30]), rendering the pyrolysis-assisted production more popular. Depending upon the type of carbon as well as the scale of reaction, these materials can be produced as films on a substrate or part [31], or as micro- and nano-scale structures that are pre-patterned employing lithographic techniques [7, 9]. The organic precursors may range from very simple molecules such as methane to a complex mixture of high molecular weight polymers and other hydrocarbons [12, 32]. Pyrolytic decomposition is also an extensively used process in the petrochemical refineries [33]. The fact that a variety of heavy hydrocarbons can be broken into smaller molecules that can be further fractionalized is utilized for decomposing the solid organic waste. Here, a mixture of organic (natural and/or synthetic) waste is often heat-treated in large-scale reactors or plants in order to produce useful chemicals [34].

A reaction is designated pyrolysis if (i) the precursor material is in the decomposition phase rather than bond formation and (ii) the cleavage of bonds is solely thermal. However, in many instances, the heat-treatment may also lead to partial bond formation along with dissociation, which cannot be clearly differentiated from pyrolysis. In certain reactions, the temperature-induced chemical modifications may not be solely thermal. For example, the presence of oxygen within the pyrolyzing precursor can lead to partial combustion of the material. Other reagents such as hydrogen may be present to dilute the precursor for prevention of excessive formation of a particular product [35]. The use of the term pyrolysis can be often found in the literature of such processes as well, which is acceptable as long as the primary decomposition mechanism is thermal. Notably, pyrolysis is different from both combustion and natural decomposition owing to its very definition.

It is evident that pyrolysis is a very versatile process, which is used in wide range of applications directly or indirectly. Unfortunately, this versatility is also responsible for the fact that this process is often studied only in the context of one specific research field [18, 36–41]. Different scientific communities may even use different nomenclature for essentially the same process. Often the connection is missing between various research fields that utilize the same principles and processes, with differences only in terms of process parameters. In this contribution, our goal is to compile a comprehensive review of the pyrolysis process that encompasses (i) its fundamental principles and mechanism, (ii) classification, (iii) process parameters and their tuning, (iv) all application areas with respective examples and finally, (iv) a comparison and possible integration of different application areas. This is particularly of interest for the development of energy materials and systems, which rely on pyrolysis in many ways.

HISTORY AND NOMENCLATURE

History

The use of pyrolysis for technological applications dates back to almost two centuries when carbon filaments for incandescent lamps were reportedly derived from cellulose fibers extracted from cotton and bamboo [26]. Hollow carbon filaments similar to CNTs were observed as early as in 1952, which were formed by the thermal decomposition of gaseous hydrocarbons in a closed retort [27, 42]. Single crystal graphite (now known as graphene) was produced by thermal decomposition of acetylene around the same time [43]. Some early primary batteries utilized during the WW-II contained pyrolytic carbon materials (e.g. charcoal) in the electrodes of the Leclanché cell [44].

Pyrolysis is also responsible for the formation of carbonaceous materials below the Earth’s crust, which is integrated with the carbon cycle. In fact, an entire branch of carbon science, Deep Carbon study, is dedicated to understanding the fate of different types of organic materials under harsh geophysical and environmental conditions [45]. These so-called deep carbons constitute approximately 90% of the Earth’s total carbon [45]. Such investigations reveal the possible pathways of formation of different carbon allotropes due to tectonic movements, sudden changes in the temperatures, meteorite impacts, high-pressure as well as other extreme conditions. Formation of graphite in the sedimentary rocks is believed to have originated from the organic matter trapped within rocks, where each pore of the rock may have served as a ‘reaction chamber’, thus facilitating pyrolysis over millions of years [46, 47]. Various carbon allotropes are present at varying depths under the Earth’s crust, depending upon the different temperature and pressure conditions experienced by the initial organic matter.

Nomenclature

The term pyrolysis should only be used to allude to chemical reactions taking place at temperature significantly higher than the ambient temperature in order to differentiate between pyrolysis and natural chemical decomposition. A chemical reaction taking place between 100°C and 300°C, for example, may simply be called thermal degradation rather than pyrolytic decomposition, which typically takes place between 300°C and 800°C. Pyrolysis is also often associated with burning. Burning is a complex combination of combustion, pyrolysis due to heat generated by combustion, evolution of volatile compounds, steam distillation, aerosol formation, etc. [48]. A clear distinction between the processes of pyrolysis and combustion during burning is extremely difficult because the formation of free radicals during the reaction with oxygen can be involved in the pyrolytic decomposition of molecules [48]. In addition, the free radicals formed from molecules due to heat (pyrolysis) can be the initiators of a combustion process [49]. Any organic material undergoing combustion at some stage undergoes pyrolysis to produce gaseous fuels to further initiate the combustion process and generally yields very small molecules like H₂O, CO₂, CO and N₂ [50]. Therefore, utmost care must be taken while differentiating between these high-temperature processes as well as their nomenclature.

In the context of polymeric carbon research, the terms pyrolysis and carbonization are often used interchangeably. Notably, pyrolysis of a polymer produces tars, gases as well as solid carbon (also see the ‘Waste treatment via pyrolysis’ section). If the intended final product is carbon, pyrolysis can be considered as the pathway to carbonization. However, carbonization is the process that entails C–C bond formation that generally takes place between 800°C and 2000°C. If the material is heated further, this region (2000–3000°C) is referred to as graphitization [51]. There are examples of pyrolysis of coal to extract tars [52], volatile organic compounds (VOCs) [53] and char (carbon with impurities) [54] where each product has its own industrial relevance. This type of pyrolysis is more common for petroleum products. As such, both pyrolysis and carbonization are thermolysis processes but with different target products. Similarly, in the case of pyrolysis of light/gaseous hydrocarbons, the overall process is known as CVD. Importantly, the first step of the CVD process is pyrolysis, which is followed by the collection, migration and deposition/growth of the desired carbon nanomaterial. Discrepancies pertaining to nomenclature of these carbon nanomaterials are also relevant for carbon scientists. However, this vast topic is beyond the scope of this article. Interested readers may find the recommendations by Bianco et al. [55] helpful.

CLASSIFICATION OF PYROLYSIS PROCESS

Pyrolysis can be classified based on (i) the phase of precursor, (ii) scale of reaction (which determines the type of reactor) and (iii) target product(s), as illustrated in Fig. 1. Based on the precursor, pyrolysis can be classified into solid, liquid and gas phase. Solid phase pyrolysis primarily utilizes synthetic and natural polymers [13, 56], solid petrochemicals such as coals and cokes [54] and hydrocarbons of mixed compositions such as biomass [40, 57] or municipal solid waste (MSW) [58, 59]. Production of mesophase carbons [a precursor for meso-carbon micro beads (MCMB), carbon foams, etc.] and the production of CF by pyrolysis of petroleum pitches [60] and naphthenic residues [61] fall under the category of liquid state pyrolysis. Notably, polymers are often in their liquid state when they are patterned or spun. But before their heat-treatment, they are typically cross-linked, dried and stabilized. Some precursors such as pitches may however be in the semi-solid state also during their heat treatment. Examples of further pyrolytic cracking of pyrolysis oil (the tarry product generated during waste pyrolysis) are also carried out with a liquid precursor [62]. Gas phase pyrolysis relies on the principle of cracking a hydrocarbon gas such as methane or acetylene at sufficiently high temperatures followed by the collection of solid carbon deposits onto a substrate. As the precursor is present in gas (vapor) phase, this entire process (pyrolysis followed by material deposition and film growth) is known as the CVD. CVD is a more general term that is also applicable to various other chemicals that yield non-carbon element or compound deposits. In the case of carbon materials, however, the precursor gas is essentially a hydrocarbon, and hence, the fundamental process responsible for the CVD is pyrolysis.

Figure 1:

Different classification pathways of pyrolysis process.

The second type of classification is based on the reaction scale and reactor type/size. Laboratory scale heat-treatment can be performed in a tube furnace, small reactors or chambers that can facilitate a controlled environment (e.g. inert gas or vacuum) [9, 11, 18, 63]. In some cases, the size of the precursor sample may be extremely small (micro or even nano-gram scale), for example, in the case of analytical pyrolysis [64] used for fossils, and in situ pyrolysis investigations performed on a transmission electron microscope (TEM) [65]. Here, the pyrolysis chamber is associated with another instrument, that may entail specially designed chips [66], wires [67] or customized sample holders [68]. Industrial pyrolysis is either used for large-scale carbon material production or for the purpose of waste treatment. In waste pyrolysis, the availability of waste determines if the process should be batch or continuous. The feed waste is often pelletized prior to pyrolysis [69]. The common reactors used for waste pyrolysis are rotary kilns [70], fixed bed [71], fluidized bed [72], tubular and certain batch and semi-batch reactors [73]. Plasma is also used for waste pyrolysis, which requires a specialized plasma reactor [73, 74]. Based on the target product, pyrolysis can be divided into three main classes: (i) carbon production, (ii) pyrolysis oil and synthetic gas production and (iii) hydrocarbon fragmentation for analytical purposes. Carbon production can be further divided into synthesis of nanomaterials, preparation of large-scale industrial carbons and carbon-conversion of polymer structures intended for device application. Details on each type of pyrolysis process will be discussed in subsequent sections.

PYROLYSIS MECHANISM

Pyrolysis typically involves covalent bond dissociation and rearrangement, which takes place between 300°C and 800°C for most hydrocarbons. The mechanism may range from simple to very complex, depending upon the nature of the precursor. For example, methane can yield some carbon species along with hydrogen slightly above the temperature where its energy of formation becomes positive [75]. A polymer, on the other hand, may exhibit complicated fragmentation patterns with parallel secondary and tertiary reactions and release volatile byproducts. Salient features of light and heavy hydrocarbon pyrolysis are described below.

Pyrolysis of light hydrocarbon

Pyrolysis of hydrocarbon gases such as methane, ethane, acetylene and low boiling point liquids such as alcohols is carried out for the purpose of carbon nanomaterial production during their CVD [35, 76, 77]. A hydrocarbon molecule disintegrates at a temperature where its free energy of formation (ΔG_f) becomes positive [75]. Since, at all temperatures, finite partial pressure of various hydrocarbons is in equilibrium with hydrogen and solid carbon, its pyrolytic disintegration can never quantitatively lead to the formation of only carbon and hydrogen [78]. The equilibrium compositions are attainable only above the disintegration temperature for a particular hydrocarbon. The carbon solubility (total amount of gaseous hydrocarbons in equilibrium with carbon and hydrogen) reaches a minimum at a certain temperature for a given total pressure of the reaction chamber [78], which plays an important role in determining the optimum process pressure as well as the type of catalyst for carbon collection. At the temperatures corresponding to this carbon solubility minima, a spontaneous decomposition of the hydrocarbon takes place. Below this, the attainment of equilibrium is very slow. Consequently, other thermodynamically unstable hydrocarbons may exist in the reaction chamber [32].

For example, at pressure ≤10⁻² bar and temperature >500°C, the cracking of methane becomes thermodynamically feasible. This leads to the formation of ‘carbon smoke’ in the chamber, which contains various carbon species including thermodynamically unstable ones (i.e. radicals, carbon moieties having two to eight atoms and some cyclic structures). Around 900°C, methane gas approaches equilibrium with these solid carbon species and hydrogen, that is carbon solubility in gas phase exhibits a minimum. Hence, even though thermodynamics suggest that methane disintegrates at temperatures >500°C), solid carbon deposits are only obtained around 900°C [32]. These carbon deposits are collected on to a catalytic substrate in the form of carbon films, tubes or other nano structures [31]. The catalyst plays an important role in determining the film growth rate, film thickness as well as the termination of reaction [79]. Further details on various catalysts are provided in the ‘Carbon nanomaterial synthesis’ section. Overall, the formation of carbon from light hydrocarbons follows three main reaction stages: (i) cracking of aliphatic hydrocarbons into smaller molecules or reactive species, (ii) cyclization of hydrocarbon chains to form aromatics and (iii) condensation of these aromatics to form polycyclic aromatics on a suitable substrate [32].

Pyrolysis of high molecular weight hydrocarbon

High molecular weight hydrocarbons include polymers, pitches, cokes and their mixtures. Their pyrolysis can be understood in terms of both chemical and physical changes, as discussed below.

Chemical aspects

During heavy hydrocarbon pyrolysis, a series of primary, secondary and tertiary reactions take place in parallel in a highly dynamic system [25, 80, 81]. The primary chemical changes that occur (generally in sequence) typically include (i) cleavage of C-heteroatom bonds to generate free radicals, (ii) molecular re-arrangement, (iii) thermal polymerization (iv) aromatic condensation and (v) elimination of H₂ from the side chains [81]. The bond cleavage is based on the bond dissociation energies (BDEs) of the specific carbon-heteroatom bond. Although these reactions take place in parallel, only one of them is dominant at a particular pyrolysis temperature [82, 83]. For example, when we pyrolyze coal, at around 300–400°C, condensable coal-tar is released along with other volatiles due to reaction type (i), but at the same time, steps (ii) and (iii) occur in the remaining solid. With increasing temperature, step (iii) becomes dominant over other two steps and char or coke is obtained around 800°C [82]. One can terminate the heat-treatment process at any temperature, allowing only few of the aforementioned steps to complete. For treatment of waste, for example, the process is terminated at step (iii); hence, the maximum pyrolysis temperature does not exceed 800°C and the final solid residue contains pores and impurities [13].

In the case of carbon material production, the process is terminated after step (v). Here, the entire heat-treatment can be divided into three stages: pre-carbonation (pyrolysis), carbonization and graphitization (optional) [84]. Pre-carbonization stage encompasses breaking of C-heteroatoms bond and re-arrangement of the C–C bonds followed by dehydration and elimination of halogens below 500°C due to their lower BDEs (mostly <450 KJ/Mol⁻¹). At this stage, a rapid weight loss is observed due to the elimination of volatiles [85] and cyclization (formation of aromatic network) [86]. Above 500°C, bonds with higher BDEs (>600 KJ/Mol⁻¹) are broken, and oxygen and nitrogen are eliminated. However, at this stage, the thermal polymerization is dominant [81] and the aromatic networks gets interconnected, resulting in primary volume shrinkage and rapid weight loss in the solid. This phase is called ‘carbonization’ stage, which takes place at temperatures >800°C [51] and may extend up to 2000°C for some polymers. It is intuitive that an organic material of high molecular weight will decompose to form carbonaceous hydrocarbons of lower molecular weights. However, it is not always the case, as some organic molecules on pyrolysis may result in molecules larger than the starting ones. For example, during the thermal cracking of n-Hexadecane (n-C₁₆) [87], along with the low molecular weight hydrocarbons, (alkanes (C₁–C₁₄) and olefins (C₂–C₁₅)), higher molecular weight alkyl hexadecanes and alkanes (C₁₈–C₃₁) are also obtained [87], which could be attributed to thermal polymerization.

Further heat-treatment above the temperature of 2000°C leads to gradual elimination of any structural defects due to aromatic condensation and the elimination of the last fragment of volatiles [81]. This stage is called the ‘graphitization’ stage, which takes place at temperatures above 2000°C [51]. Here, the crystallite diameter of residual pyrolytic carbon (L_a) is increased and the stack thickness (L_c) is decreased. An example of a heavy hydrocarbon precursor is poly-vinyl chloride (PVC), that undergoes all the three stages during its conversion into synthetic graphite [86].

Physical aspects

In terms of physical changes (e.g. phase, density and morphology), heavy hydrocarbons adopt one of the two possible mechanisms, known as coking and charring, during their pyrolysis. These principles are described in detail elsewhere [7]. Briefly, if the material experiences softening such that there is a liquid or semi-solid phase during its pyrolysis, it is said to undergo coking. Charring, on the other hand, refers to a relatively high rigidity and the protection of the carbon backbone in its nearly original morphology during and after its pyrolysis. These morphological aspects are of paramount importance when the target product is carbon. Precursors that undergo coking yield the carbon with an extremely flat surface and exhibit mostly microporosity. Charring leads to meso, macro as well as microporosity in the residual carbon.

Both physical and chemical aspects of pyrolysis are strongly influenced by (i) the highest process pyrolysis temperature, (ii) temperature ramp rate and (iii) residence (dwell) time at the highest temperature. The effect of these parameters on the composition and microstructure of the pyrolysis products is detailed in sections ‘Carbon nanomaterial synthesis’ and ‘Waste treatment via pyrolysis.’

Characterization of polymer pyrolysis

Physicochemical changes occurring during heat-treatment of a polymer can be studied by thermogravimetric analysis (TGA), differential thermal analysis (DTA) and by characterization of the material produced at different temperature points. One can also chemically analyze the volatile byproducts generated during the process via GC–MS [64]. Oils or tars can be separately collected using a condenser unit and then be further analyzed. Other characterization techniques such as elemental analysis, Raman spectroscopy, X-ray diffraction and neutron diffraction can be used for understanding the residual carbon [65, 88–92].

In the recent past, some methods for observing the microstructural changes during the heat-treatment (insitu) have also been developed. Figure 2 is a collection of TEM, TGA, XRD and wide-angle neutron scattering (WANS)/wide angle X-ray scattering (WAXS) data that indicate the microstructural changes taking place in the solid residue during pyrolysis, which ultimately converts into different types of carbon. It can be clearly observed from the TEM images (Fig. 2A and B) that between 600°C and 800°C the material undergoes major microstructural changes and its fragments display a high mobility [65, 88]. This is also supported by electrical and mechanical property tests of these intermediate materials [93]. TGA analysis (Fig. 2C) of cellulose indicates that there is a significant mass loss between 300°C and 400°C [89]. XRD data show an increased peak intensity from the (002) and (100) planes, suggesting a better order and crystallite growth in the resulting carbon with an increase in pyrolysis temperature in the range 500–900°C. It also reveals the shifting of the (002) peaks to higher angles with increasing temperatures (Fig. 2D) [90]. The exsitu WANS and WAXS data for carbon obtained from (poly)-furfuryl alcohol also confirm an increased order due to the annealing of some of the defects (Fig. 2E) [91]. Some other techniques used for insitu observations of pyrolysis include a study of planetary materials by Raman Spectroscopy integrated with Laser-heating [92].

Figure 2:

In situ TEM images of a pyrolyzing SU-8 thin-film up to 1200°C, scale bars—1 nm (A), reproduced with permission from Sharma et al. [65]; TEM images of in-situ heating of photoresist, S1805 showing migration and merging of a small graphitic domains at various temperatures (B), reproduced with permission from Shyam Kumar et al. [88]; TG–DTG curves of in-situ pyrolysis of cellulose at a heating rate of 5°C/min (C), reproduced with permission from Zhu et al. [89]; in-situ XRD studies of pyrolysis of coals at various temperatures (XRD diffractograms) (D), reproduced with permission from Li et al. [90]; (ex-situ) WANS AND WAXS data of pyrolysis of poly-furfuryl alcohol at various temperatures (E), reproduced with permission from Jurkiewicz et al. [91]. TG-DTG, thermogravimetry-differential thermogravimetry.

APPLICATIONS OF PYROLYSIS

A summary of applications of pyrolysis along with the associated manufacturing pathways is presented in Fig. 3. Table 1 contains the typical temperature range and pyrolysis environment used in these different applications. As most of the application areas are rapidly progressing, one can find some variations in pyrolysis conditions for specific cases. We have summarized the typical values here. In the subsequent sections, we review the representative examples from each application area.

Figure 3:

schematic representation of classification of applications of pyrolysis into four major areas: (A) carbon material production, (B) fabrication of carbon-micro nano devices, (C) chemical analysis of unknown samples by Py-GC–MS, (D) treatment of waste. CNF, carbon nanofibers; HOPG, highly oriented pyrolytic graphite.

Table 1:

Typical temperature range and other parameters pertaining to different applications of pyrolysis

S. No	Application area	Pyrolysis conditions	Target product and remarks	Ref.
1.	Carbon material production	600–1200°C, deposition on catalytic substrate	Carbon nanomaterials by CVD (Graphene, CNT, VGCF)	[1]
		350–600°C, in the presence of plasma	DLC coatings by PECVD	[94, 24]
		900–2800°C	Spun CFs; graphitic content in fibers is enhanced at high temperatures	[22,95]
		2500–3000°C	Highly oriented pyrolytic graphite (HOPG) can also be prepared at lower temperatures under stress; precursor: pyrolytic graphite	[5]
		2000–3000°C	Bulk GC; lower temperatures yield material with lower purity	[51]
		900–1000°C	Porous carbons that can be further activated	[96]
		500–1000°C	Mesophase carbons (pitch and coke pyrolysis)	[97]
2.	Fabrication of carbon-based micro-nano devices	900–1100°C	Precursors: high carbon containing, lithography compatible polymers	[7]
3.	Analytical pyrolysis	300–1000°C	Fragmented hydrocarbons are analyzed using Py-GC–MS	[67,98]
4.	Waste treatment	400–500°C	Almost equal proportion of char, pyro-oil, and syngas are obtained	[12]
		500–700°C	Pyro-oil, major product. Often sold as an alternative fuel (obtained from plastic waste)	[13,12]
		>700°C	Syngas, the major product	[58, 99]

Carbon nanomaterial synthesis

CVD of carbon nanomaterials such as graphene, CNTs, vapour grown CF (VGCFs), VD diamonds and DLC films is based on the principle of pyrolysis [28], where a gaseous hydrocarbon is pyrolyzed. Historically, CVD and similar processes were used for carbon production as early as the 19th century [26, 43, 100]. However, various pyrolytic carbon materials were only considered as byproducts, as the ultimate goal was to synthesize graphite. Only in the last few decades the potential of carbon nanomaterials was recognized and they were studied as independent materials. Experimental work on single (2D) crystals of graphite was reported in the 1960s [43, 101]. Prior to its synthesis, the electronic properties of this so-called 2D-graphite were theoretically studied in 1947 by Wallace [102]. Graphene-oxide, another derivative of single crystal graphite, was reported as early as 1859 [103] in a different context. The term ‘graphene’ was added to the IUPAC database in 1994, based on its experimental preparation reported in 1962 [101]. In 2004, Novoselov et al. [104] developed a novel method for obtaining graphene from HOPG by mechanical exfoliation, for which they were awarded the Nobel prize in 2010. With advances in nano-scale characterization techniques and extensive ongoing research across the globe, graphene has become one of the most technologically important materials of the 21st century. Apart from graphene, other carbon nanomaterials CNTs [105], VGCFs [21, 106] and DLC [2] are also of immense technological significance. They are also prepared via pyrolysis of gaseous or light liquid hydrocarbons. The pyrolysis conditions as well as the morphology and type of catalytic substrates may differ in these cases. Table 2 contains the standard CVD parameters for synthesis of various carbon nanomaterials. More specific details are discussed below. As there are multiple detailed review articles and books available for each individual nanomaterial, we have only provided the details of their synthesis that fit in the scope of this review. For further reading, relevant reference material is suggested.

Table 2:

Carbon nanomaterials synthesized by gas phase pyrolysis (CVD) and their process parameters

S. No.	Carbon nanomaterial	Precursor gas	Pyrolysis parameters	Ref.
1.	Single-layer graphene	CH4 + H2, C2H2, C6H6	CVD, temperature; 500–1000°C, catalyst; Cu, single crystal Ni, Cu–Ni binary alloys, thin films. He gas	[1, 18]
2.	Multi-layer graphene	CH4 + H2, C2H2, C6H6	CVD, temperature; 500–1000°C, catalyst; polycrystalline Ni thin films, He gas	[1, 18]
3.	SWCNT	CH4 + H2 CH4 + Ar, camphor (Industrial Production)	CVD, temperature; 700–1200°C, catalyst; Fe, Ni, Co, Fe2O3 nanoparticles (<10 nm)	[1, 107, 108]
4.	MWCNT	C6H6, C2H4 + N2	CVD, temperature; 550–850°C, catalyst; Fe, Ni–Cu nanoparticles	[1, 107]
5.	VGCF	C6H6, C3H8, C2H2, Ferrocene	CVD, temperature; 500–1500°C, catalyst; Fe, Co, Ni, Au, Fe–Ni, Ni–Cu nanoparticles >20 nm	[1, 21]
6.	VDDs	CH4 + H2/O2	Filament-assisted thermal CVD, Filament temperature; 2200°C Substrate temperature; 600–1200°C, Pressure; 3–4 kPa, Diamond or non-diamond substrates	[2]
		C2H2 + O2	Combustion-flame-assisted CVD, substrate temperature; 600–1200°C
7.	DLC films	CH4, C2H2	PECVD, Temperature 350–600°C (metals and alloys coatings)	[24, 94]
8.	Fullerenes	CH4 + H2	Hot filament CVD, Filament temperature; 2000–2200°C, Substrate temperature; 950–1000°C	[1, 109]
		C2H2 + Ar + H2	Microwave-enhanced CVD, Substrate temperature; 950–1000°C

SWCNT, single-walled CNTs, MWCNTs, multi-walled CNTs, VGCF, vapour grown CF.

Graphene

Graphene is defined as a defect-free single layer of graphite. This material can also be prepared via mechanical or electrochemical exfoliation of HOPG (also see the section ‘Highly oriented pyrolytic graphite’) [104]. However, CVD is a bottom-up fabrication technique that is preferred for making relatively defect-free, large area graphene films [6]. For this purpose, a gaseous precursor such as methane, ethylene and benzene along with an inert gas (e.g. He and N₂) is fed into a reactor. The precursor gas disintegrates at approximately (600–1000°C) close to the surface of a heated catalyst (transition metals in most cases). The carbon species produced by this decomposition diffuse into the metal and precipitate out onto the metal surface, leading to nucleation and subsequent growth of graphene films. The quality of graphene can be controlled by optimizing the precursor gas flow rate, inert gas flow rate, catalyst, reaction time and the pressure inside the CVD chamber which affects the activation energy required for formation of graphene nuclei on the catalyst surface [110, 111]. The formation of single-layer or multi-layer graphene depends upon the solubility of carbon and the catalyst used in the CVD process. Transition metals with unfilled d-orbitals (e.g. Co/Ni) exhibit higher affinity for carbon atoms and hence produce multi-layer graphene by dissolution and precipitation of carbon species, whereas the ones with filled d-orbitals (e.g. Cu/Zn) feature a low affinity to carbon, hence carbon diffuses onto the surface and forms mono-layer graphene [110]. Further, details on CVD reactor variations, various catalysts used, optimum temperature for graphene growth based on precursor–catalyst combination, can be found in some recent review articles on this topic [17, 112–115]. CVD graphene is primarily used for its electrical properties after transferring it from the metal substrate to other suitable substrates [116]. For a detailed information on the applications of CVD graphene, a recent article by Saeed et al. [117] may be referred.

Carbon nanotubes

CNTs have the appearance of rolled-up single or multiple layers of graphene, which are designated as single-walled or multi-walled CNTs (SWCNTs and MWCNTs), respectively. As CNTs feature a curvature, they are occasionally also considered a part of the fullerene family of carbon. Fullerenes feature a new hybridization between sp² and sp³ [118] as the unhybridized p-orbital lies at an angle between 90° (as in an ideal sp² carbon material) and 109.5° (as in an ideal sp³ carbon material). CNTs are produced by CVD, which involved pyrolytic decomposition of gaseous hydrocarbons and carbon deposition (referred to as ‘growth’ in the case of tubes and fibers) on catalytic (nano)particles rather than films. The catalyst particles may be attached to a substrate (seeding catalyst method) or float in the CVD chamber (floating catalyst method) [119] which is heated to 550–1200°C. The temperature ranges for obtaining SWCNTs and MWCNTs are listed in Table 2. The carbon precursors are mostly similar to those used for graphene (e.g. methane, acetylene, ethylene and toluene) that are introduced into the chamber at a specific rate in presence of an inert gas (Ar/N₂). Elemental carbon moieties diffuse into the catalyst and precipitate carbon either from the top or bottom of the catalyst particle. Typical catalysts used for CNTs are transition metals such as Fe, Co and Ni [120]. Their size determines whether the CNTs will be SW [121]. For SWCNTs growth, catalyst particles should be less than 10 nm [108]. Other process optimization parameters are synthesis temperature and pressure, reaction time and inert gas-flow rate [19, 122]. Sometimes CVD is carried out in presence of plasma to enhance the rates of the reactions taking place inside the chamber. Such a CVD process is termed as plasma-enhanced CVD (PECVD). It has been reported that with PECVD, CNTs can be produced at temperatures as low as 120°C [123]. Further information on CNT synthesis and applications can be accessed in some recent literature on this topic [20, 124–127].

Vapor grown CFs

VGCFs are nano-scale solid carbon filaments with an aspect ratio of around 100 [21, 106]. They are different from conventional bulk CF (having diameters of a few micrometers) in their preparation process and hence their properties. Their synthesis involves a hydrocarbon gas (such as natural gas, propane, acetylene, benzene, ethylene and methane) as the precursor, undergoing thermal decomposition in an inert atmosphere at around 950–1100°C on the surface of a catalyst, which are normally metal nanoparticles (Fe/Ni/Co), >20 nm in size [119, 128–130]. Similar to CNTs, the catalyst can be present onto the heated substrate or sometimes can be fed along with the precursor gas as floating catalyst [129, 131]. The catalyst particle takes up carbon from the supersaturated hydrocarbon gas and leaves out tubular filaments of mainly sp² hybridized carbon. The formation mechanism of VGCFs is similar to the formation of CNTs, with the difference in the size of the catalyst particles used for the decomposition of the hydrocarbons [132, 133]. It is because of the catalyst size, instead of tubular cross-section in case of CNTs, fibers with the cross-section consisting of flakes of graphite layers in various orientation precipitate out of the catalyst [134, 135]. There are reports that VGCFs having high degree of graphitization can also be prepared by CVD without the use of a catalyst on the surface of ceramic substrates [106]. VGCFs are excellent candidates as filler materials for polymer matrix composites [136–138] and carbon–carbon composites [139–143]. They are also used in energy storage devices as filler in electrodes of lead-acid batteries and Li-ion batteries, and in supercapacitor applications [144, 145].

Vapor-deposited diamonds

Carbon thin films (hydrogenated or dehydrogenated) prepared via CVD and having significant portion of sp³ carbon atoms with negligible sp² content are referred to as vapor-deposited diamonds (VDDs) [2]. The use of CVD for diamond growth started in the late 1960s ([2, 146, 147]). A breakthrough was achieved when atomic hydrogen was used for etching away the graphite deposits. This left a high content of diamond deposits on the substrate. VDDs are used as industrial coatings because of their excellent mechanical properties, especially on various cutting tools [148].

Diamond films are deposited using PECVD including filament-assisted and microwave PECVD methods [149, 150]. Plasma is required to dissociate the hydrogen molecule into reactive atomic hydrogen, which is essential for the formation of diamond instead of its thermodynamically more stable counterpart, graphite. The H atom temporarily bonds with the fourth carbon atom (in the unhybridized p-orbital) to form a tetrahedral geometry (as in the case of sp³ hybridization). This prevents the structure from forming flat sheets of trigonal planar graphite-like geometry (sp² hybridization). The temperature of the plasma can be as high as 2000°C, but substrate is maintained at lower temperatures (<1000°C). At higher temperatures >1200°C, graphite deposits is more stable. There is no specific requirement when it comes to the substrate. Often industrial machine parts are directly coated with the VDD films. For growth of single crystal diamond, however, a diamond substrate is essential, which renders the process relatively expensive [151]. For bulk poly-crystalline diamonds, silicon is the widely used substrate [6, 151]. The growth of diamond on a non-diamond substrate requires an extra nucleation step that provides the substrate with necessary diamond seeds for diamond growth. These seeds grow three dimensionally until the grain coalesces to form a poly-crystalline film [6]. The properties of CVD diamonds films have been studied and reviewed in various old and new publications [2, 152–154]. Sometimes, along with the precursor hydrocarbon, precursors of boron(B) or phosphorus(P) is also introduced into the CVD chamber to obtain B/P doped diamonds, which are used in the semiconductor industry/power electronics [155–157].

DLC films

DLC is a metastable form of carbon, which is physically amorphous in bulk but consists of small diamond-like crystallites (composed of sp³ hybridized carbon) dispersed randomly in the matrix of sp² carbon at the microscale. Hence, it is a disordered type of carbon. It features a higher fraction of sp²-content as well as hydrogen impurity (>50%) compared with VDDs [6] which differentiates the two. Both DLC and VDD are used in applications where their optical properties, high hardness and wear resistance can be harnessed [148, 158–160]. Some common examples include their coatings on the automotive parts [161], biomedical tools [159, 162], optical devices [158] and cutting tools [148, 163].

DLC film deposition requires a substantially lower (300°C) substrate temperature compared with VDD. Here, the plasma generation for dissociation of hydrogen molecule is induced by a high-frequency discharge [164, 165], which does not produce very high temperatures. Consequently, graphite deposits are not etched away by atomic hydrogen and significant sp² carbon is retained in the material. Films of up to 0.5-µm thickness can be obtained on any substrates (including polymers) [6], which is an advantage of the DLC coatings over VDD. A disadvantage of DLC films is their low temperature resistance [166] that impedes their use in high-performance thermal coatings (operating temperatures >300°C). DLC coatings also feature a high residual stress and lower toughness, that limits many mechanical applications. These limitations can be overcome by doping DLC with foreign materials such as chromium [167], nitrogen [168] and silicon [169] to form DLC nanocomposites. The source of these dopants in gaseous form can be mixed with the precursor hydrocarbon gas used for DLC deposition [168], or in solid form can be deposited on the substrate by sputtering (to form an interlayer) and DLC grown on the interlayer [167]. DLC nanocomposite with Cr doping enhances the mechanical properties by improving the fracture toughness of the material [167], N and Si doping improves thermal stability of DLC coatings and reduces the friction coefficient [168, 169]. For more details on DLC nanocomposites, readers can refer to the article by Abdul et al. [163].

Manufacture of spun CFs

Another well-known application of the pyrolysis process is the fabrication of CF from various solid/semi-solid precursors (heavy hydrocarbons). CF and CF-based composites are extensively used in the aerospace [170] and automobile industries [171]. CF-based composites are also an important candidate for construction of turbine blades due to their high strength and low weight [172]. For manufacturing CFs, first a viscoelastic polymer or pitch is spun via melt-spinning or electrospinning techniques [173]. Afterward, they are converted into carbon via pyrolysis, as discussed in the ‘Pyrolysis of high molecular weight hydrocarbon’ section. This selection of polymers for fiber fabrication is restricted to those with a good viscoelasticity. PAN, pitches and rayon are a few examples of polymers that have good viscoelasticity and hence good spinnability; therefore, they are utilized in the commercial production of CFs. The microstructure of carbon obtained from the spun polymer fibers is different from the carbon obtained from bulk polymers because of a high surface-to-volume ratio of the fibers. This facilitates an easy annealing of pyrolysis by-products such as tars and gases, as well as other structural defects during the heat-treatment. CFs (even those derived from PAN having a turbostratic structure) can typically be made more graphitic at high temperatures [174] which is not possible in the case of bulk carbons. Polymers are typically spun (using melt-spinning or electrospinning processes) prior to their carbonization/pyrolysis. Details of the spinning processes as well as polymer selection of obtaining CF can be found in many reviews [175–177]. Commercial CFs are produced mainly by carbonization of PAN-based fibers and pitch fibers. Although carbonization of many other polymeric fibers of rayon, polyvinyl alcohol and poly-esters has been attempted, they are yet to hit the market expectations [178]. Figure 4 shows the electrospinning and melt spinning process for production of spun fibers followed by stabilization and carbonization to obtain CFs. Some polymers that have been employed for CF fabrication are polyacrylonitrile (PAN) [179], phenolic resins [180] and cellulose (lignin-based fibers) [181, 182] and its derivative (Rayon) [183].

Figure 4:

Production of spun CF by different routes using different precursors. PVA, polyvinyl alcohol.

CF from polymers

Disordered carbons are hard and brittle, which makes it difficult to pull fibers out of them. The production of CF is therefore carried out by first preparing fibers using a suitable polymer and subsequently converting it into carbon via at ≥900°C. In the 1950s, rayon fibers were carbonized and used for high temperature missile applications [183, 184], but the technical breakthrough for high strength CF started in the 1960s when PAN precursor was introduced for commercial production of CFs because of its high carbon yield (approximately 50%) [185], compared with the carbon yield of rayon (approximately 30%) [184]. Nowadays PAN is the most common precursor for production of CF on a large scale due to its high carbon yield compared with other polymers and also due to that fact that the viscoelasticity of PAN can be altered/modified to produce CF of various diameters. The diameter in turn influences the graphitizability.

Electrospun PAN fibers can be converted to CF by the following steps: (i) stabilizing heat treatment at around 300°C, to prevent the precursor fibers from melting and fusion, (ii) carbonizing heat treatment at ≥900°C in an inert environment to drive off the majority of non-carbon elements, (iii) optional high-temperature treatment (≥2500°C) to improve mechanical properties of the fibers and increase the graphitic content of the fibers. Fibers undergoing steps (i) and (ii) are generally called CF and fibers undergoing all the three steps are also called graphite fibers [22]. Commercial CFs are either obtained in the form of a tow or a yarn, with each tow/yarn containing thousands of single fibers of diameter ranging from 5 to 10 µm. These fibers are either braided or woven into a mat and are commercially available as ‘preforms’. These preforms are mainly used as filler material/laminates for fabrication of polymer matrix composites [186, 187] and carbon–carbon composites [188–190].

CF from petroleum pitches

Although PAN-based CFs account for approximately 90% of the world’s CF consumption [191], the carbon yield of PAN is relatively low [185]. The search for other inexpensive raw materials as precursors for CF started in 1970s, which led to use of petroleum pitches for making precursor fibers having >70% carbon yield [192]. Their mechanical properties of pitch fibers are comparable to PAN-derived fibers and they are relatively cost effective [193].

Pitches are a byproduct of petroleum and coal processing, but can also be synthetically produced, for example, from PVC [194]. The chemical composition of pitch is very complex and is mainly a mixture of polycyclic aromatic hydrocarbons and tars. However, the composition of pitches also depends on its source [195]. Pitch-based fibers (isotropic and mesophase) [95, 196, 197] are generally processed via melt spinning to obtain pitch fibers. Pitch fibers are then stabilized/oxidized followed by carbonization to obtain CFs [198]. However, electrospinning of pitches has also been reported [199, 200]. Pitches can also be mixed with PAN to yield a composite of hard and soft CF [201]. The CF obtained from isotropic pitch and mesophase pitch is different in terms of structure, properties and nanotexture [23]. Mesophase pitch already contains small graphitic crystallites and the resulting fibers are high-performance fibers, hence and are produced commercially [194, 202, 203]. CFs from isotropic pitch are of general-purpose grade and have low modulus [203]. Pitch-based fibers are used as an alternative to PAN-based CF in various applications due to its higher stiffness. Apart from that, their electrical properties are utilized in energy storage devices. More information on pitch-based fibers and their applications can be found in the recent reviews by Liu et al. [197] and Daulbayev et al. [204].

Bulk industrial carbon production

Highly oriented pyrolytic graphite

HOPG is a synthetic graphite which is prepared by thermal and/or stress annealing of pyrolytic graphite [5]. Pyrolytic graphite is nothing but multiple layers of graphene deposited by CVD of hydrocarbons. These graphene layers are initially defect-containing and turbostratic (randomly oriented), but they organize themselves in an ABABA fashion with an interlayer spacing of <3.36 nm when heated at very high (typically 2500–3000°C) temperatures as shown in Fig. 5 (pathway A–B). When pyrolytic graphite is subjected to high temperatures and uni-axial compressive stress, the mosaic spread (angle between the tiles of graphite) is reduced. HOPG, however, is not a unique material. It is graded based on the mosaic spread. If the mosaic spread is less than 1°, it is called HOPG. Other methods to obtain HOPG include heat-treatment of polymers such as PVC, anthracene that yield graphitizing carbons [205]. One common application of HOPG is also production of graphene via exfoliation as shown in Fig. 5 (pathway B–C) from HOPG prepared by pathway A–B (Fig. 5). The exfoliation process can be physically, chemically or electrochemically assisted. Physical exfoliation methods use mechanical/ultrasonic forces (sonication) to break the weak van der waals bonds between the individual layers of HOPG and obtain graphene layers [104, 206–208]. Chemical exfoliation of HOPG generates reduced graphene oxide (r-GO) as the final product, by treating HOPG with strong acids (sulfuric/nitric acids) at a temperature slightly higher than the ambient temperature [209]. Electrochemical exfoliation methods involve intercalation of some ions electro-chemically driven in-between the layers of HOPG, leading to mesoscale mechanical exfoliation [210–212]. HOPG is used for a variety of applications including X-ray optics and spectroscopy [213, 214], anode material for Li-ion batteries [215–217] and as a substrate for thin-film deposition [218].

Figure 5:

schematic of HOPG formation from CVD graphene and graphene formation from HOPG.

Glass-like carbon

GC is a type of non-graphitizing carbon [219], that is formed by coking during its carbonization from organic precursors. Most common precursors of this type of carbon are phenolic resins [220] or (poly)-furfuryl alcohols [221]. The precursor resin is first cured/cross-linked and then heated to elevated temperatures at a very slow rate. The resins are heat treated to temperatures as high as 3000°C, to anneal out structural defects [51]. During carbonization, inter-twinning of randomly oriented graphene sheets takes place, giving rise to closed inaccessible pores. GC contains fullerene-like structures that also contribute to its low density [65]. These curved structural units make it difficult for the graphitic planes organize during further heat treatment, hence the value of L_c is always >3.36. The microstructure of this type of carbon has been studied in the past and various models proposed [42, 51, 65, 219], which reveal the short-range ordering among graphitic crystallites and randomly oriented basal planes.

GC is hard and brittle, resistant to chemical attacks and features higher tensile and compressive strength [51]. Many large-scale applications of GC-like chemical reactor linings and laboratory crucibles/substrates utilize its chemical inertness, which makes it impermeable to gases and liquids [222]. Other applications include reference electrodes for electrochemical studies [223], medical implants [224, 225] and molds for glass lenses [226]. However, production of bulk GC still has scope for optimization due to the following reasons: (i) the precursor resins used for production of GC are expensive and the high carbonization temperatures increase the overall production cost, (ii) inevitable weight loss during carbonization, (iii) difficulty in machining GC to close tolerances and (iv) difficulty in obtaining thicker (>5 mm) GC parts without porosity [51]. However, this material is studied extensively in the micro and nano-scale by photo patterning the precursor resins and carbonizing them, to obtain GC micro-nano structures, utilized for various applications, which is discussed in the section ‘Fabrication of carbon-based micro and nano devices.’

Activated carbon

Activated carbons exhibit a surface that can easily adsorb foreign molecules (liquids and gases) owing to the presence of porosity and active chemical functional groups. The gas/liquid molecules are held by weak forces (van der waals and london dispersion forces) [227, 228] that can often be released at higher temperatures or use of a chemical effluent [229]. They are prepared by physical or chemical activation of porous carbons, which are in turn obtained by pyrolysis of natural polymers. Activation process generally increases the fraction of micropores (<2 nm) and the overall surface area of the material as well create some active functional groups on its surface [230]. Porous carbons are non-graphitizing. They experience direct charring during their pyrolysis and contain fractal pore geometries (i.e. the pore sizes repeatedly decrease [231]). During its carbonization, the original skeleton of the precursor material is preserved and these types of carbons exhibit very high porosity (micro/meso/macro pores) and thus, a high surface area [229]. To produce porous carbons, the heat-treatment temperature should not be very high (typically limited to <1000°C), as higher temperatures may lead to closing or annealing of some pores [51]. Common precursors used for obtaining porous carbons include coal, petroleum residues and cellulose-based precursors (coconut shells, rice husk, wood and various biodegradable materials) [51]. Lately, a large number of agricultural and forestry residues have been utilized for the preparation of porous carbons that can be further activated. Some of these are covered in the section ‘Waste treatment via pyrolysis.’

Physical activation is done on porous carbons prepared at low temperatures, which involves heating these carbons at a higher temperature to get rid of pyrolysis by-products (tars), trapped inside the pores, thereby increasing the porosity. Another method of physical activation is to heat these porous carbona in an oxidizing environment [230]. Chemical activation is done on bio-polymers before the carbonization process by treating the precursor with some chemicals (acids/metal carbonates/metal chlorides, etc.) to partially degrade the cellulose. The polymer is then carbonized and the carbon is activated [230]. There are also many other methods of activation of porous carbons that involve combination of both physical and chemical activation processes. Interested readers can refer to the review article by Sevilla et al. [96]. Applications of activated carbons include water purification [232–234]; environmental remediation [235–237]; supercapacitor electrode material [238, 239] and as an adsorbent in food, agriculture and pharmaceutical industries [240–242].

Fabrication of carbon-based micro and nano devices

Carbon-based micro and nano devices can be fabricated using carbon nanomaterials (bottom-up manufacturing) or by directly converting a polymer structure into carbon via pyrolysis (top-down manufacturing). In this section, we will discuss representative examples of micro/nano-scale carbon structures and devices that are fabricated via pyrolysis of pre-patterned polymer structures. Such structures are often first patterned employing lithographic processes such as photolithography [11], X-ray lithography [243] and two-photon lithography [244] on to a silicon substrate, and are subsequently carbonized at temperatures ≥900° [9]. Another top-down approach that has recently gained popularity is the laser-assisted carbonization of a polymer film [10, 245, 246], which will be subsequently discussed.

Carbonization of lithographically patterned polymers

Lithography is a term used for top-down processes where a polymer film is patterned employing an electromagnetic radiation, or a high energy beam of electrons or ions. The energy of the radiation either degrades or crosslinks the exposed part of the polymer, thus modifying its chemical properties and changing its solubility. The polymers used in lithographic techniques are specifically designed for this purpose. For example, polymers that can be pattered using UV/deep-UV (photolithography) or two back-to-back photons (two-photon lithography) are able to crosslink when exposed to a pre-defined dose of the respective light due to the presence of photo-initiators moieties in their chemical structure. Interestingly, many polymers that are used in photolithography are resins that have a high carbon content and an aromatic backbone. Such polymers can yield a high fraction of solid carbon when they are pyrolyzed. This property has been widely explored for the fabrication of carbon-based devices and has been reported in various articles [7–11, 247, 248].

While converting lithographically patterned resins into carbon, the following points must be taken into consideration: (i) structures shrink due to loss of non-carbon atoms, (ii) resulting carbon is of non-graphitizing type [65, 219] which shows properties similar to commercial GC and (iii) the pyrolysis temperatures are typically limited to 1200°C, due to the fact that silicon substrates cannot withstand temperatures ≥1400°C (process temperature is kept lower for avoiding thermal stresses and fatigue). The pyrolysis temperature should also not be below 900°C, as that would yield carbon with impurities and poor electrical conductivity. Evidently, flexible polymers cannot be used as the substrate. Figure 6 is a compilation of various carbon-based micro/nano devices produced by carbonization of photo-patterned polymers.

Figure 6:

SEM images of inter-digitated carbon electrodes (A, B), the entire device (C), (A–C) reproduced with permission from Mantis et al. [248]; SEM images of sideview of optimized CNG at the edge of an electrode area (D), CNG electrodes with uniform residual bulk carbon layer connecting the CNG, (red arrows) (E), the entire device (F), (D–F), reproduced with permission from Asif et al. [249]; SEM images of suspended GCWs before and after the LCVD process (G), overview of multiple fibers suspended from scaffolds, illustrating how they can be locally coated (bright fibers) or left uncoated (darker fibers) without contaminating the carbon scaffold (H), the entire device (I), (G–I) reproduced with permission from Cisquella-Serra et al. [247]; SEM images of CMN array (J), magnified view of a CMN (K), the entire device (L), (J, K), reproduced with permission and (K) modified from Mishra et al. [250]. CNG, carbon nanograss; GCWs, glassy carbon wires; LCVD, localized CVD; CMN, carbon micro-needle.

Some representative applications of carbon structures fabricated using this process include neural sensing electrodes [11, 244, 249, 251, 252], cell culture substrates compatible with magnetic resonance imaging [8], fabrication of atomic force microscopy (AFM) tips [9, 253], biosensors [254, 255] and various other applications, which are summarized in Table 3.

Table 3:

Summary of carbon electrodes by pyrolysis of patterned polymeric structures (recent research articles from 2018 to 2021)

S. No.	Carbon structure	Proposed/tested application	Fabrication technology	Precursor polymer	Remarks, if any	Ref.
1.	3D Microelectrode	Neurotransmitter detection	Two Photon Nanolithography	SU-8	In-vivo detection of dopamine in Rat brain slices	[244]
2.	Microelectrode with suspended nanowires	Chemiresistive biosensor	Photolithography	SU-8	DNA immobilization on carbon nanowires	[254]
3.	Microelectrode	MRI	Photolithograpy	SU-8	Better MRI compatibility of GC microelectrodes compared with conventional metal electrodes	[8]
4.	Multilayer electrode	Multiple devices	Photolithography	SU-8 and Sudan III dyed SU-8	Sudan III dyed SU-8 was the sacrificial layer	[256]
5.	Microelectrode	Activation of GC microelectrodes	Photolithography	SU-8	Comparative study between electrically and chemically induced activation	[63]
6.	Microelectrode of CF mats	Neural sensors	Photolithography and RIE	PAN, PI, PDMS	Neural recording and stimultation of rat brain	[257]
7.	Microelectrode arrays	Neural sensing	Photolithography	SU-8	Flexible device on polyimide substrate for enhancing brain penetration	[258]
8.	Microneedle arrays	Drug delivery	Photolithography	SU-8	Needles tested on mouse skin without breakage	[250]
9.	Microelectrode	Hep-B antigen sensing	Photolithography	SU-8	Electrochemical sensing, LOD-1pM	[259]
10.	3D Microelectrodes	Electrochemical biosensor	Photolithography	SU-8	Amperometric glucose detection by graphene-oxide functionalized GC microelectrode	[255]
11.	GC scaffold with suspended nanowires	Localized CVD of a transition metal oxide	Photolithography, electrospinning	SU-8	Potential application for gas sensing, catalysis.	[247]
12.	3D Microelectrode	Neural sensing	Photolithography	SU-8	Flexible device on polyimide substrate folded into 3D form in origami fashion	[11]
13.	Nanograss electrodes	Dopamine sensing	Photolithography, Maskless RIE	SU-8	Electrochemical sensing of dopamine	[249]
14.	Graphene electrode	Fabrication of multi-layer graphene electrodes	Photolithography, Ni sputtering	SU-8	Pyrolytic carbon diffused into Ni and precipitates as multi-layer graphene	[260]
15.	3D microelectrodes	Interdigited electrodes	Photolithography	SU-8 (bottom layer) mr-DWL (suspended layer)	Multi-step photolithography with two resists to obtain interdigited suspended electrodes	[248]
16.	Microelectrode	Electrochemical sensors	photolithography	SU-8	CNT/SU-8 derived pyrolytic carbon for sensing of dopamine	[252]

Laser-assisted pyrolysis of polymers

A top-down fabrication technique for obtaining polymeric carbon structures is based on conversion of a high carbon containing polymer directly using a laser beam. Laser has been used in the past for production of carbon nanomaterials from thermal decomposition of hydrocarbons [261, 262], where reactants are heated by laser in a closed chamber causing the reactants to de-compose and the aggregates undergo homogeneous nucleation and growth to form hydrogen-rich carbon powders. Carbon-rich polymer films, when irradiated by a laser, undergo thermo-chemical decompositions to yield carbon structures, which can be used in micro/nano device applications. This pyrolysis is a combination of photochemical and photothermal mechanisms [263, 264]. Laser intensity is insufficient for direct bond dissociation of the polymer, but the radiation induces phonons in the material. The vibrational energy of the phonons is released by bond dissociation of the weaker components of the polymer [265]. This leads to material ablation from the surface in the form of bubbles and is expressed as ‘bleaching’ at a fluence is below carbonization threshold. Further increase in fluence leads to an immediate burst of the bubbles, resulting in rapid release of volatile products due to fragmentation of the polymer. Under constant radiation, these fragments become ionized and form a plume (plasma-like discharge). The plume-shield prevents further penetration of the beam into the material, resulting in heat generation at the beam front and adjacent areas resulting in carbonization of the material [266]. Thus, laser-induced carbonization is complete only when the plume has formed (visible as a bright spot by the naked eye).

Laser-induced carbonization has been applied successfully to polyimide [10, 245, 267, 268], parylene-C [269, 270] and polyaramid [246] to yield carbon structures, different from both glassy and activated carbons owing to the fact that this process happens within a short time and the cleavage of chemical bonds is rapid. The heat generated by the laser and the resulting carbon produced depends on the laser parameters (type of laser, laser power, speed and wavelength) along with the pyrolysis environment [10, 267]. The minimum feature size of carbon structures that can be produced by this method depends on the spot radius of the laser [270]. The microstructure of the laser-induced carbon is thoroughly investigated and applied to various applications such as supercapacitors [74, 267], sensors [10, 270, 271], antibacterial coatings [246, 272] and carbon-based composites [152]. Discrepancies in nomenclature of the same material obtained by laser-induced pyrolysis/carbonization of the same polymer are observed in the subsequent literature. For further details on laser-induced carbonization of polymers, interested readers can refer to the review article on laser-induced graphene by Ruquan Ye et al. [273]. Table 4 summarizes the recent examples of fabrication of carbon-based micro-nano devices by laser-assisted pyrolysis of various polymers and their applications.

Table 4:

Carbon patterns by laser-assisted pyrolysis of polymeric substrates (Research from 2012 to 2021)

S. No.	Structure/device	Precursor polymer	Proposed application	Remarks, if any	Ref.
1.	Microelectrode with suspended CF	Polyimide	Flexible microsupercapacitor device	Laser-induced graphene fibers (LIGF) formed from laser-induced graphene (LIG) at radiation energy >40 J/m2	[267]
2.	Porous carbon	Polyimide	Flexible on-chip micro supercapacitor	Porous foam like microstructures by femto second laser	[274]
3.	Microelectrode	Parylene-C	Neural sensing	Polymer-metal-polymer electrode with pyrolysed polymer as active site	[269]
4.	Microelectrode	Polyimide	Electrochemical pH sensor	PANI/C-PI composite electrode on a flexible substrate	[275]
5.	Microelectrode arrays	Polyimide	Neural stimulation	In-vivo cortical microstimulations in rats	[268]
6.	Graphene-like structures	Polyimide	Protection of graphene-based device from liquid erosion	Superhydrophobic graphene using a taro leaf template. Anti-sweat and serum adhesion properties	[276]
7.	Hierarchical carbon patterns	Polyimide	pH-based urea sensors	Flexible device electrochemical sensor	[10]
8.	Carbonized patterns	Polyimide	Strain sensors	Fabricated instrumented latex glove capable of monitoring finger motion in real time	[271]
9.	Microelectrode	Polyimide	Microsupercapacitors	Power supply unit for on-chip photo-detector	[277]
10.	Microelectrode arrays	Parylene C	Neural sensing	In-vitro Dopamine detection, in-vivo experiments, a future scope	[270]
11.	Carbon patterns	Polyaramid	Anti-bacterial coatings	Cu electrodeposited on flexible carbon patterns	[246]
12.	3D electrode	Polyimide	Li-ion battery electrode	Graphene transferred from PI substrate to Cu foil by rolling	[278]
13.	LIG conductive traces	Polyimide	Flexible and light-weight heaters	90°C temperature achieved at low voltages (6 V–24 V)	[279]
14.	LIG films	Polyimide	Electrochemical dopamine sensors	Graphene films formed on polyimide by irradiation of IR and UV laser	[280]
15.	LIG/PDMS/PSPI composites	PDMS and liquid polyimide	Wearable strain sensors	PDMS and liquid PI mixture spin coated and laser patterned to form a conductive path	[281]
16.	LIG/MoO2/CC electrode	Carbon cloth coated with MoO2	Microsupercapacitors	Core-shell electrode formed by laser irradiation on carbon cloth coated MoO2	[282]
17.	LIG/LDPE composites	Polyimide	Triboelectric nanogenerators	Composite formed by roll to roll	[152]
18.	Graphene mask	Polyimide	Antibacterial mask	Rapid bacteria killing by photogenerated heat	[272]

Analytical pyrolysis

Pyrolysis induces fragmentation in large hydrocarbon molecules without any foreign chemical reactions such as oxidation. This characteristic turns out to be extremely useful for the analysis of trace amounts of invaluable samples, such as the organic matter found in the fossils [14]. The analysis of fossil samples is essential for understanding their origin, age and formation mechanism. MS is one of the primary techniques used for the analysis of fossils, which is based on the principle of analyzing the mass of the various fragments of the molecule.

By evaluating this fragmentation mechanism one can detect the original structure of the initial molecule(s) [98]. Importantly, the sample quantity cannot be increased and needless to say, no amount of sample can be wasted for analytical purposes, hence pyrolysis occurs directly at the ion source to avoid loss of by-products. Pyrolysis MS (Py-MS), however, has one disadvantage that the pyrolytic fragmentation of the molecule is performed in the same chamber of the ion source. This results in contamination of the ion source, affecting long-term reproducibility of mass spectra lines [67]. Py-MS is therefore often combined with GC to form a set of techniques known as Py-GC–MS [283]. Py-GC–MS process entails the integration of pyrolyzing unit (Py), GC system and MS together by connecting the pyrolysis unit to the injector port of a gas chromatograph such that pyrolysis by-products (pyrolysates) are chromatographically separated through fused silica capillary columns by inert gas flow, followed by ionization of the products to obtain a mass spectra which is then analyzed with the help of mass spectra libraries [98, 284]. Depending upon the sample availability and its possible chemical nature, pyrolysis may be performed using ovens, lasers or by utilizing a filament that can be inductively heated to provide the desired temperatures [14]. Thus, pyrolysis may be used as a form of sample pre-treatment for analysis of complex organic materials with unknown structures [285], for example, forensic samples [14, 286, 287], humic materials [16], geopolymers [286], environmental samples [288–290], biological molecules (proteins, peptides and nucleotides) [291] and various other biochemically important polymers as well as some polymers of non-biological origin [64, 67]. A few applications of analytical pyrolysis for analysis of polymers, fossils, archaeological remains and other complex materials are listed in Table 5.

Table 5:

Applications of analytical pyrolysis methods for analysis of various complex materials

S. No.	Analytical technique	Pyrolysis parameters	Application	Ref.
1.	Pyrolysis-fast GC	Pyrolysis temperature; 700°C, pyrolysis time; 20 s	Analysis of synthetic polymers	[292]
2.	Pyrolysis-GC	Pyrolysis temperature; 820–840°C	Art and archaeology (Review)	[293]
3.	Pyrolysis-GC	Pyrolysis temperature range; 100–700°C	Investigation of humic substances in soil	[16]
4.	Laser pyrolysis-MS	Pyrolysis temperatures range; 200°C and 350°C	Characterization of biomass char	[15]
5.	Pyrolysis-GC, Pyrolysis-MS	Pyrolysis temperature range; 200–1300°C different temperature range for different studies	Biomedical studies (Fingerprinting carbohydrates, nucleic acids, bacteria, fungi, etc.) (Review)	[291]
6.	Pyrolysis-GC/MS	Pyrolysis temperature; 700°C Pyrolysis time; 10 s	Characterization of old inks from books dated 1540 and 1778	[294]
7.	Pyrolysis-GC/MS	Pyrolysis temperature; 770°C Pyrolysis time; 10 s	Study composition of weathered building materials	[295]
8.	Pyrolysis-GC/MS	Pyrolysis temperature range; 300–800°C (furnace pyrolyzer) 600–800°C (filament pyrolyzer)	Determinations of structural composition of organic matter in sedimentary rocks (kerogen)	[14]
9.	Pyrolysis-GC/MS	Pyrolysis temperature range; 50–750°C	Rapid screening of contaminants in environmental samples	[288]
10.	Pyrolysis-GC/MS	Pyrolysis temperature; 610°C	Forensic studies related to petroleum and crude oil spills	[286]
11.	Pyrolysis-GC/MS	Pyrolysis temperature; 800°C	Fast identification of polymer additives (ABS polymer from electronic industry)	[296]
12.	Pyrolysis-GC/MS	Pyrolysis temperature; 700°C	Identification of microplastics in marine litter	[289]
13.	Pyrolysis-GC/MS	Pyrolysis temperature; 700°C, 20°C/min	Characterization of lignin extracted from various archaeological sites of the world (Poland, Norway, Italy, spain, France)	[287]
14.	Pyrolysis-GC/MS	Pyrolysis temperature; 500°C (unfailed sample), 700°C (unfailed sample)	Identification of organic compounds in chemical, rubber, and automotive industry for failure analysis	[297]

Waste treatment via pyrolysis

Human, animal and plant waste contains a significant fraction of organic matter. While direct burning or combustion of waste polymers is hazardous to the environment, pyrolysis can lead to their safe disposal. The tars, gases and solid carbon residues (often called chars or biochars due to their low purity) produced during the pyrolysis of waste can also be utilized in various applications. As a result, one of the most widely studied applications of the pyrolysis process in the industry and academia at present is the treatment of waste. Here, the process is stopped at the end of the pyrolysis stage itself (generally before 700°C). The desired products are oils (tarry hydrocarbons produced by fragmentation of waste polymers) and synthetic gas (mixture of light hydrocarbons). Similar to other pyrolytic decomposition processes, waste materials which contain organic materials (biodegradable and non-biodegradable) are heated to produce the desired products. Notably, solid carbon fractions obtained at low temperatures (below 700°C) contain a significant amount of impurities and they can only be used for low-cost applications such as soil quality enhancement, oil spillage adsorbents and other industrial cleaning agents [298–300]. The quality improvement of such carbons is being extensively investigated. In the recent, past several waste-derived carbon materials have been used for advanced applications such as electrode fabrication. It is important to understand that increase in the solid carbon fraction may reduce the oil/gas production. Moreover, higher pyrolysis temperatures increase the cost of the overall process, which may not always be feasible when it comes to large-scale waste treatment. As a result, one needs to evaluate the final products prior to designing the process parameters. Various products of waste pyrolysis along with their calorific value are listed in Table 6.

Table 6:

Preparation and calorific values of the common pyrolysis products of the current waste pyrolysis facilities

S. No.	Products	Calorific value	Pyrolysis conditions	Remarks	Ref.
1	Syngas	13–14 MJ/Nm3	Final temperature: 900°C Ramp rate: 10°C/min	Produced in the range of 47–67 mol%. Production strongly depends on temperature, heating rate, and waste constituents.	[58]
2	Pyrolysis Oil	≈40 MJ/kg	Final temperature: 440°C Ramp rate: 20°C/min, Atmosphere: Nitrogen Residence time: 30 min	Sold as an alternative fuel. Higher calorific value for plastic-derived oils.	[12] [13]
3	Char	≈34 MJ/kg	Temperature: 300°C Atmosphere: Nitrogen, Residence time: 30 min	May contain heavy metals and hazardous elements like S, Cl, and Ni. Composition differs for different precursors.	[12, 301, 302]

Pyrolytic synthetic (syn) gas

The composition of the pyrolytic gas is strongly dependent on the pyrolysis temperature and feed-stock. Slow pyrolysis of biomass waste such as wood, garden waste and food residue at low temperatures (below 400°C) produces small amounts of gas, which is rich in CO₂, CO and light hydrocarbons. The yields of gas at these conditions usually do not exceed 30 wt.% of pyrolysis products. On increasing the temperature there is an increase in gas yields, because of the secondary reactions and partial char decomposition. The calorific value of gas from slow pyrolysis is around 10–15 MJ/Nm³ and varies with temperature and heating rate [303]. Fast pyrolysis of biomass produces gas with a calorific value of around 14 MJ/Nm³. On the other hand, higher temperatures (above 700°C), especially when pyrolysis is combined with gasification, produces syngas, which contains more hydrogen and carbon monoxide. In this case, syngas is the main product of the process. The pyrolysis of plastics produces pyrolytic gas, of which the major components are hydrogen and light hydrocarbons: methane, ethane, ethene, propane, propene and butane. This gas has a significant calorific value, for example, a heating value of gas from PP and PE varied between 42 and 50 MJ/kg [304]. Similar properties characterized the gas from the pyrolysis of tyres or other artificial products like textiles. In turn, co-pyrolysis of polymers and biomass leads to a higher production of CO and CO₂ especially at lower temperatures. Finally, the pyrogas from MSW consists of CO₂, CO, hydrogen, methane and other light hydrocarbons with an average heating value of around 15 MJ/Nm³, which increases with increasing temperature [305]. The most suitable demand on pyrogas is its use as a source of the energy required for the pyrolysis process itself. However, the exhaust gas has to be controlled. Pyrogas from tyres contains a relatively high concentration of H₂S, which can be oxidized to SO₂ [306]. PVC pyrolysis produces huge amounts of HCl [307] whereas waste food processing could be a source of dangerous nitrogen compounds [308]. Usually the precise composition of waste is unknown, thus some unwanted compounds can appear in pyrogas. Therefore, emission control units and gas cleaning devices should be used and it does not matter whether the gas will be combusted or not.

Pyrolytic oil

Pyrolytic oil offers more opportunities for use than syngas, but the composition of the liquid product from pyrolysis may differ radically depending on the composition of the feedstock and the process parameters. Pyrolytic oils derived from biomass consist mainly of the following compounds: acids, ketones, aldehydes, sugars, alcohols, phenols and their derivatives, furans and other mixed oxygenates. Phenolic compounds are mostly present in high concentrations (up to 50 wt.%), consisting of relatively small amounts of cresols, xylenols, phenol, eugenol and much larger quantities of alkylated (poly-) phenols [309]. It can be used for the production of heat, electricity, synthetic gas or chemicals. The highest yields of oil are gained between 500°C and 600°C. Pyrolytic oil from biomass has calorific values of around 15–20 MJ/kg, on the other hand, pyrolytic oil from plastics has a higher calorific value, about 30–45 MJ/kg, depending on the precursor polymer. Ahmad et al. [13] compared the oils from the pyrolysis of PP and HDPE with gasoline and diesel via physical properties such as viscosity, the research octane number and the motor octane number, as given in Table 7. Pour point, flash point or diesel index could be a good indication of pyrolytic oil quality as a fuel [13, 71]. The calorific value of oils from mixed plastic waste could be estimated at 40 MJ/kg [310].

Table 7:

Comparison of pyrolytic oil from some polymers with standard liquid fuels, reproduced from Ahmad et al. [13]

Properties	HDPE	PP	Gasoline	Diesel
Heating value (MJ/kg)	40.5	40.8	42.5	43.0
Viscosity at 40°C (mm2/s)	5.08	4.09	1.17	1.9–4.1
Density at 15° (g/cm3)	0.89	0.86	0.780	0.870
Research octane number	85.3	87.6	81–85	–
Motor octane number	95.3	97.8	91–95	–
Pour point	5	9		6
Flash point	48	30	42	52
Diesel index	31.05	34.35	–	40

Pyrolytic char

Currently, pyrolysis conditions are generally optimized in order to maximize the liquid and gas products. Besides these two, a solid fraction named as pyrolytic char is also produced. Char mainly is carbon-rich matrix containing almost all the inorganic compounds present in the wastes with a significant amount of condensed by-products of the pyrolysis process [311]. Chars are generally porous and its porosity depends upon precursor waste [7]. The calorific value of char obtained from pyrolysis of waste (mixture of biodegradable and non-biodegradable) is approximately 34 MJ/kg [302], which is comparable with coal. However, despite all the separation techniques before pyrolysis, some heavy metals and other hazardous elements, like S, Cl and Ni, get retained in the solid products. Therefore, it becomes equally important to characterize chars so as to assess their impact on the environment and humans. In general, this product can be combusted to provide energy for the pyrolysis process or other applications as listed in Table 8.

Table 8:

Applications of waste-derived pyrolytic carbon

S. No	Products	Precursor material	Remarks (if any)	Ref.
1.	Battery electrode	Biomass (various)	Review article	[312]
2.		Rice husk	Lithium ion batteries	[313]
3.		Bamboo	Lithium ion batteries	[314]
4.		shaddock peel	Sodium ion batteries	[315]
5.		Coffee grounds	Sodium ion batteries	[316]
6.	Supercapacitor electrode	Rice husk	RHC: KOH = 1:5 by mass was used for activation	[317]
7.		Carrot	ZnCl2 as activation agent	[318]
8.		Biomass (various)	Review article	[319]
9.		Coconut shell	–	[320]
10.		Tobacco	–	[321]
11.		Tamarind fruit shell	Activation of char was done by treating the precursor with KOH	[322]
12.	Dye sensitized solar panel (counter electrode)	Filter paper facial tissue	–	[323]
13.		Fish waste	Pt-free counter electrode	[324]
14.		Coconut shell	Anthocyanin dye extracted from pomegranate juice	[325]
15.		Anchovy	–	[326]
16.	Water filtration or adsorbents	Biomass (various)	Review article	[327]
17.		Nutshells (Almond, English Walnut, Pecan)	–	[328]
18.		Apple pulp	Adsorption of lead and zinc	[329]
19.		Fertilizer waste	Heavy metal removal from fixed bed reactor	[330]
20.		Chickpea	–	[331]
21.		Municipal organic solid waste	–	[332]
22.		Coconut button	–	[333]
23.		Municipal sewage sludge	–	[334]
24.		Human hair	Sensor for dopamine and ascor-bic acid	[335]
25.		Polyethylene terephthalate (PET) bottles	Detection of carbofuran phenol	[336]
26.		Amla	Sensor for ascorbic acid, dopamine, uric acid and nitrite	[337]
27.		Onion peel	Sensor for progesterone	[338]
28.		Biomass (various)	Review article	[339]
29.		Tetra pak waste	Mercury adsorption from water	[340]
30.	CF manufacturing	Cotton	–	[341]
31.		Prepreg fibers	Prepreg is ‘pre-impregnated’ composite fibers	[342]
32.		Prepreg fibers	70% strength when compared with new fibers	[343]
33.	Filler material for fiberglass/epoxy composites	Metallized food packaging plastic waste	0.75 wt.% of char particles improved the modulus of elasticity of panels by approximately 22%, compared with a pure sample.	[344]
34.	Hybrid fillers for cement industry	Textile waste	Char particles (0.05 wt.%) was used as a single filler and hybrid fillers (CNTs/CB and graphene/CB: 50/50) to enhance the properties of cement paste	[345]

Catalytic pyrolysis

Various catalysts are used during waste pyrolysis that can potentially increase the oil, gas or char fractions, as desirable in the process. Catalytic pyrolysis of plastic waste is typically carried out in presence of natural/modified zeolites to produce pyrolysis oils, which can be used as transportation fuel by mixing or blending with conventional fuels [346, 347]. Other catalysis that are being extensively studied include metal oxides and bimetallics [152]. More information about catalytic pyrolysis of MSW and plastic wastes can be found in these recent articles [347–349].

Semi-cokes/mesophase carbon from pyrolysis of pitch

It is not possible to commercially exploit all of the crude petroleum of the barrel for commercial purposes and the various distillation and cracking processes produce a huge amount of residues within the refinery, the disposal of the same is of major concern [350]. These residues are rich in aromatics and has high C/H ratio, hence can be a good feedstock for mesophase carbon. When these residues are heat treat at around 450°C, they convert into a pitch-like isotropic material having a consistency similar to liquid crystals. With increasing temperature, small spheres appear in the pitch-like mass, which grow with time. At some stage in the heating process, the spheres will replace a large part of the pitch-like material and interfere with one another’s enlargement and a ‘mosaic’ begins to form by coalescence when all of the isotropic pitch-like material is replaced by the anisotropic material or mesophase and the mosaic is complete, the mesophase solidifies into ‘semi-coke’, which is readily graphitizable [351]. With further heat treatment (1400°C), this carbonaceous mesophase coalesces to a state of bulk mesophase before solidification to ‘green coke’ with further loss of volatile compounds [350]. However, apart from this regular trend, many different behaviors have been observed for varying compositions of the feedstock. The petroleum residues are a mixture of more than 1000 molecular compounds (numbers differ in literature) and contain mixtures of aliphatic and aromatic compounds. To obtain high-quality cokes acceptable for industrial usage (as electrodes for steel industry), high aromaticity in the precursor is essential. Before getting converted to green coke at higher temperatures, this pitch material, known as mesophase pitch can be a good precursor for preparation of high-performance carbon materials [352] like pitch-derived coke [353], mesocarbon microbeads (MCMB), CF [3, 23, 194], carbon foams and carbon composites [354]. A summary of various carbon forms obtained from mesophase pitches and their respective applications is listed in Table 9.

Table 9:

Different types of carbon materials obtained by pyrolysis of mesophase pitch and their applications

S. No.	Carbon form	Precursor source	Pyrolysis parameters	Remarks (if any)	Ref.
1.	MCMB	Co-pyrolysis of coal tar pitch and direct coal liquefaction residue s	Temperature; 440°C, 8 h, N2 gas.	Li-ion battery, anode material	[3]
2.		Naphthalene isotropic pitch	Suspension method, HF/BF3 catalyst, temperature; 1000°C	Li-ion battery, anode	[355]
3.		Petroleum pitch	Carbonization temperature; 400°C, 4 h, N2 Gas, heat treatment in vacuum, 380°C, 30 min	Li-ion battery, anode material	[356]
4.		Coal tar pitch	Carbonization temperature; 700°C, 2 h, N2 gas	Sodium-ion battery, anode material	[357]
5.		Catalytic cracking oil residue	Temperature; 350°C (30 min) 400°C (40 min) 400°C (4–9 h)	Investigation of the relationship between olefins and the coalescence of mesophase spheres	[358]
6.	Carbon foams	Coal pitch	Carbonization temperature; 450°C, N2 gas	Investigation of acoustic properties	[359]
7.		Coal tar pitch	Carbonization temperature; 600°C, 2 h.	Thermal insulation	[360]
8.		Coal tar pitch modified with HNO3 and H2SO4	carbonization temperature; 2000°C, N2 gas	Microstructure investigations	[361]
9.	Hierarchical porous carbons	Coal tar pitch,	Final temperature, 800°C, 2 h, Ramp rate, 2°C/min anhydrous aluminium chloride (catalyst)	Supercapacitor electrode	[4]
10.	MCMB/carbon foams	Coal tar pitch	Final temperature; 440°C, N2 gas, 12 h, foaming pressure, 3 MPa	Investigation of microstructure and properties	[362]
11.	Activated carbons	Petroleum residues (decanted oil and ethylene tars)	Temperature; 400–460°C, 2 h,4 h, and 6 h, respectively	Methane adsorption	[363]
12.	Mesoporous soft carbons	Napthalene-based synthetic pitch	temperature; 350–800°C, 5°C/min, 2 h, N2 gas	Anode material, sodium-ion batteries	[364]
13.	Needle coke	Coal tar pitch	Temperature; room temperature to 1400°C, 5°C/min, 1 h holding time, N2 gas	Green coke with aromatic index of 0.95–0.98 obtained.	[365]
14.	Graphene	Petroleum mesophase pitch	Activated carbon: temperature; room temperature to 800°C, holding time; 2 h,	Graphene was prepared from mesophase pitch derived activated carbon by exfoliation (treated with N-methyl-2-pyrrolidone [NMP])	[366]
15.	N doped mesoporous carbon	Mesophase pitch and polypyrolle (N source)	Final temperatures; 600°C, 700°C, 800°C, 10°C/min, N2 gas	Supercapacitor electrode	[367]

Special cases

Although the term pyrolysis is predominantly used in the context of organic materials, there are certain examples where pyrolysis is performed on inorganic solids/liquids as well. Synthesis of 2D nanomaterials like graphitic carbon nitride from pyrolysis of urea [368], molybdenum sulfide by CVD [369] and thin films by CVD of inorganic precursors [370] are a few examples. Another variation of pyrolysis known as spray pyrolysis [371–373], in which precursors in liquid phase are sprayed through an atomizer onto a heated substrate (250–500°C) [374], mainly aimed at deposition of thin and thick semiconductor films for solar cells applications [375–377]. However, at present, this process has been extended to deposition of thin films for sensors [378], solid oxide fuel cells [379] applications as well. Spray pyrolysis technique has also been utilized in the synthesis of various nanomaterials apart from thin films [380–382] which is beyond the scope of this paper due to vastness of the topic. Another term known as ‘hydropyrolysis’, that is, pyrolysis in presence of hydrogen at high pressure, is predominantly performed on biomass to obtain biofuels/chemicals for industrial use in presence of a catalyst. Hydrogen is used as a reducing agent to form hydrogenated radicals by reacting with the volatiles and to remove oxygen in the form of water, CO and CO₂, resulting in hydrocarbon generation [383]. However, hydropyrolysis in itself is a vast topic and is beyond the scope of this paper.

CONCLUSION AND WAY FORWARD

Pyrolysis is extensively used in different applications that are covered in this review in terms of their fundamental principles, history, industrial relevance and process parameters. Evidently, these applications not only belong to entirely different scientific communities, their target products and production scales also widely vary. One important conclusion is that pretty much all synthetic carbon materials, bulk or nano-scale, are derived from organic precursors via the pyrolysis process. Given the significance of advanced carbon allotropes in the cutting-edge technology, there is a compelling need for (i) reducing the cost of pyrolysis, (ii) improving the efficiency of the process and (iii) development of integrated pyrolysis systems. Lowering the energy consumption during pyrolysis is not straightforward, but is possible with the use of sophisticated nano-scale catalysts that can potentially lead to an overall cost reduction. One challenge is to get rid of the catalyst particles through post-processing with a high yield, which demands more focused research. Generally, catalysts can also facilitate an increase in the overall process efficiency. While the idea of efficiency may differ based on the application area, tuning of the underlying process parameters can always be of help. For this purpose, a comprehensive understanding of the pyrolysis mechanism for a given precursor is essential. Key concepts pertaining to this are covered in detail in this review.

The development of the integrated pyrolysis systems may serve multiple purposes. Plenty of work has lately commenced in this direction, where the goal is to further pyrolyze the byproduct(s) of one pyrolytic process. A good example is carbon nanomaterial production via secondary pyrolysis of the synthetic gas obtained during waste pyrolysis. Such innovative ideas need technological support from both academia and industry, for example, an optimized reactor design suitable for the quantity of the feed. Based on the information available in the literature, such multi-stage pyrolysis equipments are already proving to be extremely helpful in improving the commercial viability of the waste treatment plants. Some other integrated processes such as the microbial bioprocessing of pyrolytic oils have also lately gained attention for the generation of fuel with a higher calorific value. An additional future prospect is the quality enhancement of low-grade biochars by increasing the pyrolysis temperature and ensuring a strictly inert environment during the process. The know-how is already available with the activated carbon industry and researchers are rapidly coming up with very promising results. For large-scale pyrolysis, plasma-assisted processes and/or solar energy supported plants are also recommended. The age-old process of pyrolysis is expected to play a major role in the near future in the carbon materials science as well as the expansion of the sustainable energy solutions.

Acknowledgments

M.D. would like to thank the Ministry of Education, Government of India, for her doctoral fellowship. S.S. acknowledges the financial support from the Seed Research Grant No. IITM/SG/SWS/69, Indian Institute of Technology, Mandi.

AUTHORS’ CONTRIBUTIONS

M.D. prepared the initial draft including figures and tables as well as contributed to editing and finalizing the manuscript. S.R. contributed in drafting the waste pyrolysis section. S.S. conceptualized, edited and finalized the manuscript.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.